Semi-conductors have been known in the industry for many years and the term semi-conductor material has been considered generic to a number of materials, including silicon. As used herein, the term is intended to pertain to the silicon elements or "chip" semi-conductor substances. Customarily, these elements are in the form of a wafer or disc. They may be circular, rectangular or triangular or any other convenient shape. Silicon is characterized in an electron energies diagram by a relatively wide gap between the top of its valence band and the bottom of its conduction band. This inherent property of silicon makes possible stable electron operation at relatively high temperature and also results in low reverse currents across a P-N junction region of such a body. As a consequence, silicon shows considerable promise for use in semi-conductor devices, particularly where they are to operate at high temperatures such as when dissipation effects associated with large currents being handled cause appreciable heating of the semi-conductor body or in situations where it is important to have as low reverse currents as possible in the semi-conductor device.
Generally, the silicon element has an active impurity incorporated therein which impurity affects the electrical rectification characteristics of the silicon as distinguished from other impurities which may have no appreciable effect on those characteristics. Active impurities are usually classified as donor impurities or acceptor impurities. The donor impurities are phosphorus, arsenic and antimony and the acceptor impurities are boron, gallium, aluminum and indium.
With respect to the nomenclature used in the semi-conductor art, a region of semi-conductor material containing an excess of donor impurities and yielding an excess of free electrons is considered to be an impurity doped N-type region. On the other hand, an impurity doped P-region is one containing an excess of acceptor impurities resulting in a deficit of electrons or an excess of holes. In other words, an N-type material is one characterized by electron conduction whereas a P-type material is one characterized by hole conduction. When a continuous solid specimen of semi-conductor material has an N-type region adjacent to a P-type region, the boundary between them is termed a P-N or N-P junction and the specimen of semi-conductor material is termed a P-N junction semi-conductor device. The present invention is concerned with a P-type silicon element which has formed thereon a phosphorus-containing layer which is an N-type region. The reverse side of the silicon chip or wafer retains its P-type nature and accordingly, the product produced by this invention is a P-N junction semi-conductor device. Therefore, to make a simplified P-N junction requires the addition of N-type impurity to a surface layer of P-type semi-conductor.
Semi-conductors have application and utility for purposes such as rectifiers, transistors, photodiodes, solar batteries, semi-conductor controlled rectifiers and other devices. In addition to general electronic applications, the P-N junction semi-conductor is frequently used as a radiation detector or charged particle detector. For example, a charged particle such as a proton, alpha, or electron releases some of its energy in passing through the P-N junction, and produces an electrical pulse which is amplified and which is proportional to the energy of the particle. In this particular usage, it is quite important to have a thin uniform P-N junction which is made possible by the present invention. Also, electromagnetic radiation such as visible light or particularly infra red radiation may be detected by its interaction with the P-N junction.
Silicon devices containing a diffused P-N junction have been made by heating P-type silicon chips or wafers in the presence of a phosphorus compound such as phosphorus pentoxide. The phosphorus pentoxide is believed to form a glassy film over the surface of the wafer and subsequently, with continued heating, elemental phosphorus diffuses into the silicon. The phosphorus could also be deposited on the surface of the silicon wafer at a low temperature and then heated to a temperature at which diffusion will take place.
Various developments have taken place in the prior art to effect the doping of the semi-conductor material by the addition of dopant impurities while the silicon crystal is being pulled from a melt or by applying alloying and diffusing methods to a growing crystal. In general, the diffusion of the doping substance into the silicon material is effected by heating a predetermined quantity of the particular substance together with the silicon in a closed receptacle so that the dopant atoms will permeate from all sides into the semi-conductor body. Methods involving deposition of a dopant on a limited surface area of the semi-conductor body are described in U.S. Pat. No. 3,287,187. This prior art method requires the deposition of an oxide of the semi-conductor material by vapor deposition followed by diffusion of the doping substance into the semi-conductor surface area by heating the semi-conductor body.
Another method of diffusing phosphorus oxide into a semi-conductor crystal is shown in U.S. Pat. No. 3,540,91, wherein the source of the phosphorus compound is produced by fusing an alkaline earth phosphate and phosphorus pentoxide. Illustratively, the fusion product of tertiary calcium phosphate and phosphorus pentoxide is used and it is said to yield reproducible results for doping semi-conductor crystals of silicon. However, when a mixture of calcium phosphate and phosphorus pentoxide is used containing appreciable phosphorus pentoxide, the material would be a molten glass or at best a molten mass at normal doping temperatures ranging from 900.degree. C. to 1200.degree. C. This would necessitate containing the phosphate mixtures or salts in a boat or crucible in a temperature zone typically lower than that required for diffusion after the doping process takes place. Two different temperature zones or two different reaction conditions would thereby be involved if this particular prior art method were followed. The requirement for containing molten materials in a boat or crucible drastically reduces the number of silicon chips which can be treated simultaneously in a uniform temperature zone and further complicates the procedures.
Another approach described in the prior art for doping a wafer of semi-conductive silicon involves the use of vapors of ammonium phosphate. U.S. Pat. No. 2,974,073 discloses a method wherein the vapors of ammonium phosphate are employed and is reported to form a glassy phosphorus containing surface film over the wafer. It is said that some phosphorus diffuses from the film into the wafer to form a phosphorus diffused N-type surface on the wafer. Commercially used doping temperatures commonly fall within the range from 900.degree. C. to 1200.degree. C. and at these temperatures, the ammonium phosphate would decompose completely rendering it useful for only a single doping. These properties would necessitate temperature zones typically lower than that required for diffusion after the doping process takes place. The U.S. Pat. No. 2,974,073 shows that a convenient method of forming the step is to utilize a two-zone furnace. The requirement for maintaining two different temperature zones or two different reaction temperatures places an unwanted burden on the commercial practice of this method.